A) Field of the Invention:
The present invention relates to an achromatic lens system which is to be used in optical systems for telescopes, microscopes, cameras, video cameras, etc., and an image pickup lens system which comprises an achromatic lens system.
B) Description of the Prior Art
An optical system which is to be used in telescopes, microscopes, cameras, video cameras, etc. generally has a composition in which a large numbers of lenses are combined for enhancing optical performance of the optical system. A cemented achromatic lens which is referred to as an achromat is frequently used in optical system for correcting chromatic aberration in particular. This achromat corrects chromatic aberration ordinarily for rays having two wavelengths such as the C-line and the F-line, but does not strictly correct chromatic aberration for rays having different wavelengths such as the g-line, thereby allowing chromatic aberration which is referred to as the so-called secondary spectrum to remain. This residual chromatic aberration often poses problems in objective lens systems for telescopes and microscopes as well as telephoto lens systems for cameras and so on in particular, and a lens system which is made achromatic for three wavelengths and is referred to as an apochromat is used for correcting the residual chromatic aberration.
However, it is necessary for manufacturing apochromats to use fluorite or low dispersion glass materials which have extraordinary dispersion characteristics, and are expensive and hard to work, thereby enhancing manufacturing costs of optical systems.
In addition to ordinary lenses which utilize the refraction phenomenon, there are available lenses which utilize the diffraction phenomenon and are referred to as diffraction type lenses. The diffraction type lens has a dispersion characteristic which is different from that of the ordinary lens. As an example wherein a diffraction type lens is used for correcting the secondary spectrum of chromatic aberration, there is an optical system which consists of a combination of a cemented lens and a diffraction type lens as described in Applied Optics, Vol. 27, pp 2960 through 2971.
Furthermore, as another example of lens system wherein chromatic aberration is corrected using a diffraction type lens, there is known a lens system such as that disclosed by Japanese Patent Kokai Publication No. Hei 4-181908 which uses a combination of a diffraction type lens and a radial type GRIN lens.
An optical system such as that described in the literature mentioned above (Applied Optics) which consists of a combination of a cemented lens and a diffraction type lens uses three optical elements in total or a large number of optical elements, thereby requiring a high manufacturing cost. Moreover, the lens system disclosed by the published bulletin mentioned above which consists of the radial type GRIN lens and the diffraction type lens is designed while paying attention only to chromatic aberration of the first order and no description is made of the secondary spectrum in the bulletin.